The proposed study is intended as a continuation of a current project investigating the phonological knowledge and learning patterns of young children with functional (nonorganic) speech disorders. Research techniques and findings from linguistic theory, speech science and speech pathology are combined in an interdisciplinary investigation of disordered sound systems. The proposed study used descriptive and experimental techniques for the analysis of cross-sectional and longitudinal data. There are two large scale projects proposed, each with a different emphasis. Project I includes two experimental treatment studies that manipulate linguistic structures and principles in the explanation of changes in individual phonological systems. The first study is designed to integrate three factors (i.e. implicational laws, stimulability and acoustic differentiation) which have independently been shown to contribute to change phonological systems. Forty-eight phonologically disordered children would receive treatment based on the implicational relationship of the treatment target relative to the occurring sounds in the phonetic inventory, stimulability of excluded sounds and acoustic differentiation of seemingly homophonous sounds. Treatment will be provided on one specific sound defined by a combination of the three parameters in order to predict on an a priori basis changes in each individual's phonological system. The second study examines the role of rules and representations (underlying and phonetic) in phonological learning. Thirty-six speech disordered children will receive treatment on one sound associated with correct or incorrect underlying representations,k with correct or incorrect phonetic representations or with the (non)application of a phonological rule. The relative ease/difficulty of overcoming problems associated with these phonological structures and principles will be assessed. Project II intends to extend in various ways our analysis of the 55 speech disordered subjects from the current grant. Three studies are planned that involve post hoc analyses of generalization learning patterns and unexplained changes in phonetic inventories. The first study examines in detail the generalization learning patterns of treatment sounds in relation to specific words to determine whether treatment effects are uniform across different words involving the same treatment sound. The other two studies in this project reexamine seemingly anomalous changes in phonological systems to determine whether recently discovered implicational laws or finer-grained acoustic differentiation will account for those changes.